Structural Biochemistry/Metals

Importance of Metal Ions in Biochemistry

Metal ions play a significant role in the biological systems especially when the ion has empty or half filled d-orbitals. They can positively and negatively affect metabolic processes. Most metals have a strong affinity to accept electrons and form bonds. Thus, a lot of proteins fold into a structure that and bond the metal to form a function(e.g., Hemoglobin). With the help of iron, hemoglobin transports dioxygen O₂. Also, iron (Mg²⁺) holds an essential role in the formation of deoxyribonucleotides by ribonucleotide reductase. Iron in its complexes transforms between two oxidation states by transferring one electron from this state to another state, such as the ferrous (Fe²⁺), and ferric (Fe³⁺). By the process of one electron transfer between two states of iron and storing them, an organism can be able to survive in an aerobic world; however, they should maintain this form of iron in soluble and being ready to use which prevents from presence of uncontrolled redox chemistry.

There are two important proteins that help organisms performing these activities:

a. Transferrin: the iron-transport protein

b. Ferritin: the iron-storage protein.

Trace metal ions are important in biochemical systems. More than one-third of all enzymes require the addition of metal ions or contain a bound metal ion. A protein that contains a metal cofactors is called a metalloprotein[1]. Metal ions have a few characteristics that increase chemical activity: positive charge, ability to form strong bonds that are kinetically favored, and in some cases their ability to be stable in multiple oxidation states.

Many enzymes require metal ions for catalytic activity. Carbonic anhydrase was the first known enzyme to contain zinc, but since then, hundreds have been discovered. A few years after the discovery of Carbonic Anhydrase, scientists discovered this enzyme not only contained a bound zinc ion but that the ion was necessary for catalytic activity. For more information on metal cofactors, see Metal Ion Catalysis .

Carboxypeptidase A contains the Zn⁺² metal in the active site

Less common trace metals are chromium, nickel and vanadium. Other ions are often toxic to organisms, such as barium, beryllium, cadmium, lead, arsenic, selenium, aluminum, thallium and tin. [2] A deficiency of certain metals, such as Zinc(II), have been shown to lead to slowed growth, impaired metabolism and developmental retardation (Ashley & Ridgway, 1970).

Metal ions and Its Role in Bacteria

Bacteria like with any organism has evolved and learned to find their own niche. There has been discovered bacteria that may utilize metal ions for energy. This bacteria or microorganism is known as the Geobacter metallireducens. What it can do is take oxidize organic substances and utilize that to couple with the reduction of metal ions.

The very first of its species was found in the Potomac River in 1987, very close to the nation's capital Washington D.C. This was the first microorganism found that was capable of oxidizing organic compounds into Carbon Dioxide with iron oxidizes (Fe(III)) assisting it as the electron acceptor. This electron acceptor is found on a structure of the bacteria known as the pilus. This added a new perspective on the role of pilus as before it was thought to only for sex conjugation, anchoring to a particular niche, and for twitching motility. This goes to show that metals, which were once thought not to be able to be utilized by any organism, is known to capable of using by bacteria.

Bioremediation

What is bioremediation? It is defined as using microorganisms to help clean up the environment or the contaminated environment back to what it used to be.

With this unique property of the Geobacter species, research has been ongoing in using the Geobacter to assist helping with ameliorating the environment. For instance, the Geobacter may take petroleum contaminants that are in dirty water and converting it into innocuous carbon dioxide by the process of oxidization. Because of their special characteristic, Geobacter are able to speed up the breakdown of pollution in the water and other places. Moreover, it can be used to take away radioactive metals from groundwater.

Not only can be used for bioremediation, Geobacter may also be used as fuel cells by conversion of organic products into electricity. This is made possible by their property of transferring electrons unto the electrode surfaces.

References

1. The Geobactor Project at University of Massachusetts Amherst. 2009. 17 Oct. 2009. <http://geobacter.org>
2. <http://www.ncbi.nlm.nih.gov/sites/entrez?Db=genomeprj&cmd=ShowDetailView&TermToSearch=192> 3. Bertin, Ivano. Biological Inorganic Chemistry: Structure and Reactivity. 2007.